The present invention relates to filters and to a method and apparatus for manufacturing filters, and relates particularly, but not exclusively, to microwave filters and a method and apparatus for manufacturing microwave filters.
Microwave filters are often constructed from networks of coupled passive resonators, each passive resonator having a finite unloaded Q factor. In narrow bandwidth applications, the resistive loss associated with this finite unloaded Q factor can lead to significant reduction in achievable performance, and in bandpass applications, designs with a good input and output reflection coefficient will exhibit significant bandpass loss variation.
In the narrow band bandstop case the resistive loss manifests itself as a roll off of insertion loss into the pass band, and also limits the achievable notch depth. The combination of these two effects limits the achievable selectivity from a bandstop filter designed using previously available techniques.
In an existing bandstop filter, resonators are coupled off from a main through transmission line with an electrical separation of an odd number of quarter wavelengths, as shown in FIG. 1. Each resonator couples loss into the system, giving rise to the problems outlined above.
In various applications of microwave filters, such as in base stations for cellular telecommunications, the above difficulties are addressed by using components having very high Q factors, typically up to 40,000. However, this increases the physical size of the devices involved, whereas it is usually desirable in such applications to make the devices as compact as possible.
Preferred embodiments of the present invention seek to provide a filter which, although constructed using finite Q elements, does not suffer from a reduction in selectivity as a result of resistive losses caused by these finite Q factor elements.
Preferred embodiments of the present invention also seek to achieve a desired filter characteristic using components having lower unloaded Q factor than in the case of the prior art.
Preferred embodiments of the present invention also seek to provide a bandstop/pass filter having a steep transition between the stop and pass band and using lower value unloaded Q factor components than in the case of the prior art.
According to an aspect of the present invention, there is provided a method of designing a filter, the method comprising defining a desired filter characteristic, and applying an algorithm to the desired characteristic to provide a filter having infinite Q factor elements and having a theoretical characteristic corresponding to the desired characteristic transformed to compensate for the difference between finite Q factor and infinite Q factor elements.
According to another aspect of the present invention, there is provided a method of manufacturing a filter, the method comprising the steps of designing a filter according to a method as defined above, and constructing using finite Q factor elements a filter corresponding to the theoretical filter.
This provides the advantage of a filter design technique which takes resistive losses of the individual components, such as inductors and-capacitors, of the filter into account, and therefore enables a filter having a desired characteristic to be designed using finite Q value components. This in turn enables a filter having a particular characteristic to be realised using lower unloaded Q factor components than in the case of the prior art, which in turn enables the filter to be constructed more compactly than in the case of the prior art.
According to another aspect of the present invention, there is provided an apparatus for use in manufacturing filters, the apparatus comprising an input means in which a desired filter characteristic is defined in use, and means for applying an algorithm to the desired characteristic to provide a filter having infinite Q factor elements and having a theoretical characteristic corresponding to the desired characteristic transformed to compensate for the difference between infinite Q and finite Q factor elements.
According to a further aspect of the invention, there is provided a filter manufactured according to a method or using an apparatus as defined above.
This has the advantage of enabling the realisation of a filter having lower Q value components than in the case of the prior art, which in turn enables the construction of a more compact filter.
According to a further aspect of the invention, there is provided a filter comprising first and second resonators interconnected by a quadruplet of impedance inverters, a ladder network connected to the quadruplet of impedance inverters via a series resistor and comprising a plurality of further resonators, wherein adjacent further resonators of the ladder network are coupled to each other by respective impedance inverters.
In a preferred embodiment, the filter is a reflection mode filter.
The filter is preferably a microwave filter.
A filter may be a bandstop and/or a band pass filter.
Preferably, the step of applying said algorithm comprises shifting the pole/zero plot of the desired filter characteristic by a constant amount.